Dry conditions persisted through March with much of Arizona receiving monthly precipitation amounts between 10 and 45 percent of average. Seasonal precipitation remains near average in the Salt River Basin and along the western Arizona border, primarily due to a wet December. Elsewhere, seasonal precipitation has dropped to below average levels. Snowpack in Arizona remains only at highest elevations, with future melt contributing little to seasonal volumes. Streamflow was above average in the Little Colorado in March due to snow melt. Elsewhere throughout Arizona March streamflow volumes were below average.

Streams are in recession from the winter season, or already near baseflow. April-May is climatologically a drier period and typically streamflows do not increase much prior to the summer monsoon. Volumetric forecasts for the April-May period are expected to range from near 20 to 50 percent of median.

Model Soil Moisture/Observed Streamflow - Streamflow volumes in March were below average for the first time this winter. Some streams are flowing near baseflow at this time.

Snowpack/Precipitation - March precipitation amounts ranged from near 10 to 35 percent of average in the Salt, Verde, and Tonto Basins. Seasonal precipitation is near average. Only minimal snowpack remains at highest elevations of the Salt River Basin and is not a significant factor contributing to runoff expectations.

General Discussion/Forecasts - April-May is climatologically a drier period in Arizona prior to the summer monsoon. Streams are forecast to continue their recession or remain near their baseflow conditions. April-May volumes in the Salt, Verde, Tonto streams are forecast to be near 40 percent of median.

Model Soil Moisture/Observed Streamflow - Streamflow volumes in March in the Gila Basin ranged from near 20 to 40 percent of median. Several months of below average streamflow combined with below average precipitation suggest soil moisture deficits exist in this area.

Snowpack/Precipitation - Snowpack has been nearly depleted in the Gila Basin. March precipitation ranged from near 20 to 50 percent of average. Seasonal precipitation is near 65 percent of average.

General Discussion/Forecasts - Seasonal volume forecasts in the Gila Basin reflect the dry conditions that exist in the Basin. April-May runoff volumes are forecast to range from 20 to 40 percent of median.

Model Soil Moisture/Observed Streamflow - Streamflow volumes for March were much above median on some of the headwater locations, such as the Little Colorado near Lyman Lake, and Chevelon Creek. Elsewhere streamflow volumes were below median.

Snowpack/Precipitation - Snowpack was above average in the Little Colorado southern headwaters on March 1st. By early April much of the snowpack had melted off, resulting in enhanced streamflow on streams draining these areas. Precipitation during March ranged from mostly 5 to 40 percent of average. Seasonal precipitation is near 75 percent of average.

General Discussion/Forecasts - Streamflow volumes in the Little Colorado Basin are expected to range from 20 to 50 percent of median for the April-May period.

Differences between the full period forecasts and the residual forecasts may not exactly equal the actual observed volumes due to rounding conventions (see Definitions section).

10% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 10% chance of being exceeded. Previously referred to as "Reasonable Maximum Forecast".

50% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 50% chance of being exceeded. Previously referred to as "Most Probable Forecast".

90% exceedance forecast: Given the current hydrometeorological conditions, i.e current snowpack, soil moisture and streamflow, the volume that has a 90% chance of being exceeded. Previously referred to as "Reasonable Minimum Forecast".

Acre-Foot (af): The volume equal to one acre covered one foot deep (43,560 cubic feet). See kaf below.

Average: The arithmetic mean. The sum of the values divided by the number of values. Values from 1971-2000 are used for this publication.

Categories: Much Above Average=Greater than 130%, Above Average=111-130%, Near Average=90-110%, Below Average=70-89%, Much Below Average=Less than 70%.

CBRFC: Colorado Basin River Forecast Center.

Forecast Period: The period from April 1 through July 31, unless otherwise noted.

kaf: Thousand Acre-Feet. See Acre-Foot above.

Inflow: The volume of water that flows into a reservoir or lake.

Median: The middle value of an ordered set of values. Half of the values are higher and half of the values are lower. When the set contains an even number of values the median is the average of the two middle numbers.

NOAA: National Oceanic and Atomospheric Administration.

NWS: National Weather Service.

Water supply forecasts take into consideration present hydrometeorological conditions and use average basin temperatures and precipitation for the forecast period. As the forecast season progresses, a greater portion of the future hydrologic and climatic uncertainty becomes known and monthly forecasts become more accurate. For more information on the tools we use, consult Water Supply Forecasting Tools.

Volume forecasts represent adjusted flows; that is, observed flows with upstream water use taken into account. Adjusted flows will closely approximate natural or unimpaired flows. However, not all upstream diversions or impoundments are measured or quantifiable. For specific adjustments used with each forecast point, consult the Guide to Water Supply Forecasting.

The Water Supply Outlook is issued monthly January through May by the Colorado Basin River Forecast Center. It represents a coordinated effort between the National Weather Service, Natural Resources Conservation Service, Bureau of Reclamation, U.S. Geological Survey and local water district managers.

Note: Data used in this report are provisional and are subject to revision.

For more information, or to be included on the mailing list, please contact:

Colorado Basin River Forecast Center
2242 W North Temple
Salt Lake City, UT 84116
(801) 524-5130
www.cbrfc.noaa.gov